The present invention relates to chemical separation processes, and, more specifically, to an improved process for separation of aromatic compounds from mixtures of aromatic and non-aromatic compounds and methods for retrofitting existing equipment for same.
Aromatic petrochemicals, such as benzene, toluene and xylenes (collectively, xe2x80x9cBTXxe2x80x9d), serve as important building blocks for a variety of plastics, foams and fibers. Traditionally, these fundamental compounds have been produced via catalytic reformation of naphtha or through steam cracking of naphtha or gas oils, producing streams such as reformate and pyrolysis gasoline. BTX derived from such traditional methods typically include substantial amounts of non-aromatic compounds having similar boiling points, effectively precluding simple distillation as a means of separation of the aromatic from the non-aromatic.
Accordingly, a variety of extraction techniques have been developed in an effort to separate aromatic compounds from non-aromatic ones. Such prior art extraction techniques typically involve the use of solvents which exhibit a higher affinity for the aromatic compounds, selectively extracting the aromatic compounds from the mixture of aromatic and non-aromatic compounds. An example of such prior art extraction techniques is the sulfolane process developed by Shell Oil Company. The sulfolane process employs the use of tetrahydrothiophene 1,1 dioxide (or sulfolane) as a solvent and water as a co-solvent. The process uses a combination of liquid-liquid extraction and extractive stripping in a single, integrated design.
Despite its wide-spread use, the sulfolane process suffers from several disadvantages imposed by its design. For example, such process is restricted in its available production capacity. This is due to the fact that in order for liquid-liquid extraction to occur, a phase separation must take place between the solvent/extract and the raffinate material. The maximum aromatic content of the feedstock is restricted to approximately 80%-90%.
Additionally, in traditional sulfolane process designs, the range of feedstock choices is limited. This is due to the fact that existing sulfolane extraction units were constructed when feedstock was presumed to include total aromatic concentrations of from about 30%-60%. With improvements in new catalysts and the development of continuous catalytic regeneration (xe2x80x9cCCRxe2x80x9d), the aromatic content of reformate streams is significantly higher, exceeding the point where liquid-liquid phase separation, and hence simple extraction, can occur. One attempt to resolve this dilemma has been to artificially recycle non-aromatic or raffinate material in order to lower the total aromatic concentration and thus promote phase separation. Alternatively, a co-solvent composition can be increased in an effort to increase the solvent system selectivity. Both of these attempts to accommodate recent developments in catalysts and catalytic systems with prior art designs significantly decrease operating efficiency and unit capacity of the process.
Another drawback associated with the prior art sulfolane process is the concentration effect of undesired components present in the reflux stream. Extraction solvents have group selectivity favoring extraction of aromatics greater than naphthenes/olefins greater than paraffins and a light/heavy selectivity which favors lower carbon number components. Accordingly, the sulfolane process design was premised upon the theory that the extractive stripping operation would easily remove lighter non-aromatic compounds, which would flow as reflux to the main extractor and displace heavier aromatics.
In practice, the design produces at least two undesired effects: (1) difficulty in recovering the heavier aromatics into the extracted stream; and (2) buildup of light impurities in the extractive stripper and reflux system. The former undesired effect associated with such prior art designs is the incapacity of such designs to completely remove and recover the heaviest species of aromatic compounds within the mixed feedstock. For example, an operation using the prior art design and processing a BTX range feedstock may result in nearly complete benzene recover while losing upwards of 15% or more of the xylenes within the feedstock into the raffinate due to the lower affinity of the solvent for xylenes compared with benzene. Such results require the employment of additional recovery schemes in an effort to more completely recover the xylenes present in the feedstock.
The latter undesired effect results in significant increases in the concentration of lower carbon number components (e.g., 5 and 6 naphthenes and olefins) within the reflux stream, which can lead to product contamination of the lowest carbon numbered aromatic compounds. Attempts to cope with this problem include increased efforts by the operator to strip such undesired components into the reflux stream and/or employing a drag stream from above the aromatic product fractionator to recycle to the extraction section. Both attempts result in increased energy consumption by and reduced capacity of the system.
Thus, there remains a need for a recovery process and method for retrofitting existing recovery process equipment to improve upon prior art aromatics recovery processes, and to avoid the disadvantages described above.
In accordance with the present invention there is provided an improved process for separation of aromatic compounds from mixtures of aromatic and non-aromatic compounds and a method for retrofitting existing equipment for employing said improved process. In one aspect, the improved process for separation of the present invention includes an extractive distillation operation as a primary separation step for the recovery of aromatic compounds. This embodiment of the invention is preferably used with feedstocks containing BTX fractions, but it is noted that it can also be used with feed fractions containing between 5 and 12 carbons.
It was discovered that the prior art sulfolane process and accompanying system suffered primarily in its design and implementation with respect to three main areas: (1) the main extractor; (2) the extractive stripper; and (3) the extract recovery operation. Although other incremental improvements were made to other aspects of the prior art process, the main improvements described herein are realized in these three primary areas.
In a first embodiment of the improved separation process of the present invention, a hybrid extraction/extractive distillation system is employed. A portion of the mixed hydrocarbon feedstock is routed to a new, separate extractive distillation column (xe2x80x9cEDCxe2x80x9d) which operates in parallel with the main extractor, extractive stripper and water-wash operations of the process. The use of an EDC allows recovery and purification of aromatic compounds to occur in a single operation. The optional use of a co-solvent further improves the recovery capability of this embodiment of the improved aromatics recovery process of the present invention.
In a second embodiment of the improved aromatic recovery process of the present invention, the hydrocarbon feedstock originates from a heartcut fractionation column (xe2x80x9cHFCxe2x80x9d), such as a reformate splitter column. Additional advantages of the process are realized by segregating the feedstock fractions to the extraction and extractive distillation operations. Use of a co-solvent may be practiced with this embodiment of the improved aromatics separation process of the present invention to further improve recovery of aromatic compounds from the feedstock.
In a variation of the second embodiment described above, a side cut of the feedstock including a heavier fraction is taken from the prefractionator column and processed in the EDC. The overhead portion is fed to the traditional liquid-liquid extraction portion of the system. The chief advantage associated with this variation of the second embodiment is the more complete recovery of heavier aromatic compounds, avoiding maximum aromatic limitations associated with prior art designs and more fully described above.
In a third embodiment of the improved aromatic separation process of the present invention, the hydrocarbon feedstock is routed directly to the EDC for processing. The overhead material is subsequently condensed and routed to the liquid-liquid extractor, which functions in this embodiment as a raffinate extractor. Of practical importance is the fact that this embodiment can make use of a modified extractive stripping tower as the EDC.
In further accordance with the invention, the improved aromatic separation process can be derived by retrofitting an existing sulfolane-based extraction system. The retrofit is accomplished by converting the original liquid-liquid extraction column into a vapor-liquid service and utilizing it as the top portion of an EDC. The extractive stripping column of the prior art system is used as the lower portion the EDC. Other elements of the prior art system (e.g., water-wash column) can be eliminated. Importantly, the hydraulic capacity of redesigned system will exceed the original capacity of the original system.
In yet further accordance with the improved aromatic recovery process of the present invention, a prior art design glycol-based extraction system can also be retrofitted to employ the improved aromatic recovery system. To accomplish this retrofit, fresh hydrocarbon feedstock is fed into the EDC tower (rather than the main liquid-liquid extractive column) along with lean solvent. The overhead stream from the EDC contains the non-aromatic compound and can bypass the traditional water-washing step. The liquid-liquid extraction column is converted to a liquid-vapor distillation service. The bottom streams from the EDC are routed to the liquid-vapor distillation service and further processed. The overhead extract product is routed directly to product tanks without any additional washing steps.
In yet further accordance with the improved aromatic recovery process and method of retrofitting existing equipment for same, an improvement of the extractive distillation process is obtained by converting original vessels used in the liquid-liquid extractive system into a raffinate extractor, a new EDC, a raffinate water-wash device and an extract recovery operation.
The primary benefits derived from the above-identified embodiments of the improved aromatics recovery process and method for retrofitting existing equipment for same, and variations thereof, can be summarized as follows:
The embodiments and variations thereof utilize either a stand-alone extractive distillation operation or a hybrid combination including liquid-liquid extraction to provide process gains, such as capacity and recovery;
All of the embodiments and variations thereof described herein operate without an aromatics (drag) stream or raffinate recycle;
Each of the embodiments and variations thereof described herein utilize an extractive distillation operation with highly effective solvents and selective addition and/or control of the co-solvent ratio, if present, within the process;
Many of the embodiments and variations described herein thereof segregate the feedstock and intermediate product streams to gain advantage over limitations present in existing equipment and to improve unit efficiency;
Many of the embodiments and variations thereof described herein allow for the liquid-liquid extractor operation to be by-passed without shutting down the system to accommodate maintenance work;
Many of the embodiments and variations thereof described herein can be implemented upon a relatively short interruption of the system so that process tie-ins and other retrofitting operations can be performed;
All of the retrofit embodiments and variations thereof described herein realize from between a 20% and 100% increase in capacity when compared with the original configuration with a minimum of reconfiguration;
Many of the embodiments and variations thereof described herein segregate the process streams and direct them to the most desirable processing operation, providing greater recovery of both light and heavy aromatic compounds;
All of the embodiments and variations thereof described herein optimize conditions for recovery, thus lowering associated operating costs when compared with traditional system designs;
All of the embodiments and variations thereof described herein more fully utilize the liquid-liquid extraction operation, thus requiring less solvent inventory when compared with prior art process designs; and
All of the embodiments and variations thereof described herein maintain high levels of purity of the lowest boiling point extracted fraction more easily due to the avoidance of the recycle and associated undesired accumulation of light-weight impurities from the liquid-liquid extraction operation.